Innate T cells learn to find their activating ligands in the skin during their thymic education using cholesterol byproducts Summary Aberrant skin inflammatory disorder such as atopic dermatitis (AD) is a rampant disease among children of developed countries, with as many as a quarter of them afflicted. AD is known to be a disease of allergic T cells that manifest when the skin barrier becomes degraded. There has been progress in identifying genes involved in tissue barrier fortification. People with mutations in these genes are predisposed towards allergic diseases mediated by lymphocytes, but most individuals with genetic susceptibility still do not develop the diseases, suggesting that there exist immune mechanisms to dampen harmful inflammatory responses. We discovered that an absence of dermal innate T lymphocytes programmed to secrete IL-17 and IL-22 results in spontaneous AD in mice with all major hallmarks of human AD, placing these cells called T??17 as the apex regulator of skin homeostasis. Murine dermal T??17 cells develop only during neonatal stages and they are made in the thymus so that they can respond fast when they seed the skin. However, how these skin regulatory cells localize to the dermis is not known. In this project we will test the hypothesis that developing T??17 cells learn to localize in the relevant skin sites in the thymus. This thymic education occurs by restricting maturation of T??17 thymocytes that can sense cholesterol byproducts oxysterols via the G protein coupled receptor (GPCR) GPR183 (Ebi2) and/or the CCR6 chemokine receptor ligand CCL20. In non-inflammatory settings, such as in healthy neonates, GPR183+CCR6+ T??17 cells exit the thymus and enter the skin using CCR6 and then fine-localize in the dermis using the putative oxysterol gradient. This is a high risk and high reward project, designed to establish the critical importance of oxysterols for innate T cell positioning in the body. The conceptual basis is unique as there are no precedents for the GPCR- dependent maturation of lymphocytes in the tissues of origin that are intimately linked to the ability of mature lymphocytes to anticipate the same cues in the tissues of function. This lack of precedent makes the project risky, but if proven to be accurate will fundamentally alter our understanding of mucosal innate T cell sensing of environmental alterations and expand the functional domains of cholesterol and its byproducts in building and maintaining a healthy immune system, especially in newborns.